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  JS Review_four.md
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<li><p>JS this关键字</p>
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<p>a.面向对象语言中，this表示当前对象的一个引用，但在JS中this不是固定不变的，它会随着执行环境的改变而改变，具体如下：<br>1 在方法中，this表示该方法所属的对象<br>2 如果单独使用，this表示全局对象<br>3 在函数中，this表示全局对象<br>4 在函数中，在严格模式下，this是未定义的<br>5 在事件中，this表示接收事件的元素<br>6 类似call()和apply()方法可以将this引用到任何对象上<br>注意：this只有在函数调用的时候才会发生绑定，谁调用了函数，就绑定谁<br>b.显式函数绑定<br>在JS中函数也是对象，对象则有方法，apply和call就是函数对象的方法。他们允许切换函数执行的上下文环境(context),即this绑定的对象。<br>例子</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> person1 = &#123;</span><br><span class="line">     fullName: <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">         <span class="keyword">return</span> <span class="keyword">this</span>.firstName + <span class="keyword">this</span>.lastName;</span><br><span class="line">         &#125;;</span><br><span class="line"> <span class="keyword">var</span> person2 = &#123;<span class="attr">firstName</span>: <span class="string">'John'</span>, <span class="attr">lastName</span>: <span class="string">'Doe'</span>&#125;  ;    </span><br><span class="line"> person1.fullName.call(person2);  <span class="comment">//返回John Doe</span></span><br><span class="line"> <span class="comment">//就是让函数在指定对象上调用，this在调用的时候发生绑定，指向person2</span></span><br></pre></td></tr></table></figure>
<p>c.关于this，分享一个我以前做过的挺有意思的一个题,:</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//下面这段JS程序的执行结果是什么</span></span><br><span class="line"><span class="keyword">var</span> user = &#123;</span><br><span class="line">count: <span class="number">1</span>,</span><br><span class="line">getCount: <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line"><span class="keyword">return</span> <span class="keyword">this</span>.count;</span><br><span class="line">&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> func = user.getCount;</span><br><span class="line"><span class="built_in">console</span>.log(func();</span><br></pre></td></tr></table></figure>
<p>我先将结果告诉你，是undefined，给你5分钟思考。<br>好了，我来解答：<br>我上面说过，this只有在函数调用的才会发生绑定，谁调用的绑定谁。那么是谁调用的这个函数呢？其实题目绕了一个弯子，它声明了一个变量接收user对象的对象方法getCount,很明显func是一个函数类型的变量，这个变量是在哪儿声明的呢？显而易见，函数外部，在函数外部声明的变量是什么？是全局变量，全局变量是属于哪个对象的？默认是属于window对象的，所以下面函数的调用func(),就等同于window.func(),这样的话，this肯定就绑定window对象了。然后我们看里面执行的代码，this.count就等同于执行window.count,这样一个语句就相当于是声明了一个变量但是没有赋值，这样的变量打印出来肯定就是undefined，如果里面this，就是执行return count,这就相当于直接使用一个没有声明的变量，程序肯定就会报错了<br>如果window.count没有懂，不妨看看下面这个例子:</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> person = &#123;</span><br><span class="line">name : <span class="string">'John Doe'</span></span><br><span class="line">&#125;</span><br><span class="line">person.age = <span class="number">10</span>; <span class="comment">//JS中是支持这样给一个对象添加一个属性的，这句话就是声明添加一个属性并赋值</span></span><br><span class="line">person.sex;         <span class="comment">//这就相当于声明了一个属性不赋值</span></span><br><span class="line"><span class="built_in">console</span>.log(person.sex)  <span class="comment">//undefined</span></span><br><span class="line"><span class="comment">//window对象是一个JS预先已经定义好的对象，这下懂了吧</span></span><br></pre></td></tr></table></figure></li>
<li><p>JS let和const<br>a.let声明的变量拥有块级作用域({}内)<br>b.const声明一个只读的常量，一旦声明，常量的值就不能改变，同样也是块级作用域<br>c.var声明的变量不具备块级作用域，在块内重新声明变量也会重新声明块外的变量，let不会</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> x = <span class="number">10</span>;</span><br><span class="line">&#123;<span class="keyword">var</span> x = <span class="number">2</span>;&#125;   <span class="comment">//x最终输出2</span></span><br><span class="line"><span class="keyword">var</span> y = <span class="number">10</span>;</span><br><span class="line">&#123;<span class="keyword">var</span> y = <span class="number">2</span>&#125;; <span class="comment">//y最终输出10</span></span><br></pre></td></tr></table></figure>
<p>d.在函数体内(外)使用var和let关键字声明的变量类似，作用域都输局部(全局)的<br>HTML代码中使用全局变量：<br>使用var关键字声明的全局作用域变量属于window对象<br>使用let关键字声明的全局作用域变量不属于window对象<br>e. var存在变量提升(hoisting),支持先使用后声明，let不行<br>f. const声明的变量必须初始化<br>g. const的本质：const定义的常量并非真正的常量，并非不可变，它定义了一个常量引用值。使用const定义的对象或者数组，其实是可变的，从这里我们可以得出：const声明的变量不能改变的是它的地址引用，而这个地址上的内容我们是可以修改的。比如：</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> car = &#123;<span class="attr">type</span>: <span class="string">'Fiat'</span>, <span class="attr">model</span>: <span class="string">'500'</span>, <span class="attr">color</span>: <span class="string">'white'</span>&#125;;</span><br><span class="line">car.color = <span class="string">'red'</span>;  <span class="comment">//这个改变时允许的，而且是可以奏效的</span></span><br><span class="line"><span class="comment">//car = &#123;...&#125;;  //数组是一样的,不写了，能偷懒就偷懒</span></span><br></pre></td></tr></table></figure>
<p>h. const不存在变量提升</p>
</li>
<li><p>JS JSON<br>a. JSON是用于存储和传输数据的格式，通常用于服务端向网页端传递数据<br>b. 什么是JSON，全拼是(JavaScript Object Notation) JS对象表示法</p>
<pre><code>JSON是一种轻量级的数据交换个事
JSON是独立的语言
JSON易于理解</code></pre><p> c. JSON语法规则<br> 数据为键/值对<br> 数据由逗号分隔<br> 大括号保存对象<br> 方括号保存数组<br> d. JS中的两个方法<br> JSON.parse() 将JSON字符串转成JS对象<br> JSON.stringify() 将JS值转成JSON字符串</p>
</li>
<li><p>JS void<br>a. JavaScript: void(0)的含义<br>void操作符指定要计算一个表达式但是不返回值<br>void(alert(‘xxx’)) //不返回值，但是括号内的表达式还是要执行<br>b. href = “#” 与 href = “javascript: void(0)”的区别<br>#包含了一个位置信息，默认的锚记是#top也就是网页的上端，而javascript:void(0)仅仅表示一个死链接</p>
</li>
<li><p>JS代码规范(老生常谈，一次没记)<br>a. 变量使用驼峰命名法<br>b. 通常在运算符前后添加空格<br>c. 4个空格缩进<br>d. 以分号结尾<br>e. 每行代码字符数小于80</p>
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            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>


<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
      tex2jax: {
          inlineMath: [ ['$','$'], ["\\(","\\)"]  ],
          processEscapes: true,
          skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
      }
  });

  MathJax.Hub.Queue(function() {
      var all = MathJax.Hub.getAllJax(), i;
      for(i=0; i < all.length; i += 1) {
          all[i].SourceElement().parentNode.className += ' has-jax';
      }
  });
</script>

<script src="https://cdn.jsdelivr.net/npm/mathjax@2.7.6/unpacked/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<script>
  var ayerConfig = {
    mathjax: true
  }
</script>



<script src="/js/busuanzi-2.3.pure.min.js"></script>



<script type="text/javascript" src="https://js.users.51.la/20544303.js"></script>


    
  </div>
</body>

</html>